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Water use efficiency (WUE), the amount of assimilated carbon per amount of transpired water is central to assess how trees and forests respond to the ongoing climate changes and, particularly, to increasing drought severity. We concurrently evaluated leaf- and ecosystem-scale intrinsic WUE (WUE i) of a Mediterranean evergreen oak (Quercus ilex) forest in southern France using stable carbon isotopic discrimination, leaf gas exchange and eddy covariance fluxes. Nine years of continuous daily eddy fluxes were converted into 7-day composite chronicle of WUE i and then into δ13C input to check, is there a link between eddy and bulk leaf δ13C? The relationship between ecosystem-scale WUE i and water limitation, assessed by the predawn tree potential (ψ pd), followed a trapezoidal curve with a plateau value of 158.0 ± 4.0 µmol CO 2 mol-1 H 2 O. WUE i began to decline from -2.4 MPa with gas exchange ceasing at -4.4 MPa. WUE i measured at leaf scale followed a bi-linear relationship with ψ pd that peaked at ψ pd = -3.15 MPa, reaching 120.5 ± 16.4 µmol CO 2 mol-1 H 2 O. Current year leaf δ13C is linearly related to the averaged δ13C input over the leaf expansion period. In 1-year old leaves, adjustment in bulk δ13C was dependent on the degree of water limitation. Whole-isotopic mass balance at tree or ecosystem scales continue to be a major challenge because large gaps remain in understanding all the processes that influence δ13C of fluxes and C pools. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhiza (AM) and ectomycorrhiza (ECM) are the two most common mycorrhizal types and are paid the most attention to, playing a vital common but differentiated function in terrestrial ecosystems. The leaf carbon isotope ratio (δ13C) is an important factor in understanding the relationship between plants and the environment. In this study, a new database was established on leaf δ13C between AM and ECM plants based on the published data set of leaf δ13C in China's C3 terrestrial plants, which involved 1163 observations. The results showed that the differences in leaf δ13C between AM and ECM plants related closely to life forms. Leaf δ13C of ECM plants was higher than that of AM plants in trees, which was mainly led by the group of evergreen trees. The responses of leaf δ13C to environmental changes were varied between AM and ECM plants. Among the four life forms, leaf δ13C of ECM plants decreased more rapidly than that of AM plants, with an increase of longitude, except for deciduous trees. In terms of the sensitivity of leaf δ13C to temperature changes, AM plants were higher than ECM plants in the other three life forms, although there was no significant difference in evergreen trees. For the response to water conditions, the leaf δ13C of ECM plants was more sensitive than that of AM plants in all life forms, except evergreen and deciduous trees. This study laid a foundation for further understanding the role of mycorrhiza in the relationship between plants and the environment. [ABSTRACT FROM AUTHOR]

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